LinQ In Action 学习第四章

第四章开始了 LinQ to Object 的讲解（Entity 用了很多linq的语法，如果Linq 掌握好了Entity很好学了）：

第四第五章 需要多加练习例子，熟练linq 的语法。

第四章的例子做完了，主要讲了一些常用的Linq 语法。

1.select select many

2. orderby 

3. group, join group, left join, cross join, inner join, nested query

4.where .......

 5. take() skip()

写了几个小例子：

 var publishersource = from bookbb in books
                                   select new
                                   {
                                       Name = from pub in Publishers
                                              where pub.Name == bookbb.Publisher.Name
                                              select pub,
                                       bookbb.Title,
                                       bookbb.Price
                                   };
             var groupsource = from groupbook in books
                               group groupbook by groupbook.Publisher into PublisherGroupName
                               select new { Name = PublisherGroupName.Key.Name, Books = PublisherGroupName };

             var groupjoinsource = from publishersor in Publishers
                              join booksor in books
                              on publishersor equals booksor.Publisher into PublisherGroup
                              select new { Name = publishersor.Name, Books = PublisherGroup };

             var leftjoingroup = from publisher in Publishers
                                   join booksor in books
                                   on publisher equals booksor.Publisher into GroupPublisher
                                   from book in GroupPublisher.DefaultIfEmpty()
                                   select new { Publisher = publisher.Name,
                                                Book = book == default(Book)?"no books":book.Title };
             var crossingJoin = from publisher in Publishers
                                from book in books
                                select new
                                {
                                    Name = publisher.Name,
                                    Title = book.Title
                                };

第五章主要讲性能：

1. nongeneric collection need to : Cast, ofType,  explicitly convert to object.

2.分组使用多个元素  group by more than one characters.